GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 545 



humidity of our climate; whilst the reindeer, formed to support 

 the rigours of a long and rude Lapland winter, suffers from 

 heat at St. Petersburg, and in general sinks quickly under 

 the influence of a temperate climate. From this it results 

 that, in a great number of cases, differences in climate alone 

 are found to be sufficient to arrest the march of species from 

 high latitudes towards the equator, or from equatorial regions 

 towards the poles. The influence of temperature on the ani- 

 mal economy explains to us also why certain species remain 

 cantoned in a chain of mountains without being able to spread 

 abroad into analogous localities. We know, in fact, that the 

 temperature decreases by reason of the elevation of the soil, 

 and that in consequence animals which live at considerable 

 elevations could not descend into the low plains to reach 

 other mountains, without traversing countries where the tem- 

 perature is much superior to that of their ordinary habita- 

 tion. The genus llama, for example, abounds in the grassy 

 countries of Peru and of Chile, situated at an elevation of four 

 or five thousand metres (from four to five thousand yards) 

 above the level of the sea, and extending to the south as far 

 as the extremity of Patagonia ; but they are to be found neither 

 in Brazil nor Mexico, because they could not arrive there with- 

 out descending into regions too hot for their constitution. 



The nature of the vegetation and of the pre-existing fauna 

 in a region of the globe equally influences its appropriation 

 by exotic species. Thus the dispersion of the silkworm is 

 limited by the disappearance of the mulberry above a certain 

 degree of latitude ; the cochineal cannot spread itself beyond 

 a region where grows the cactus ; and the large carnivora, 

 unless they live on fish, cannot exist in the polar regions, 

 where the vegetable productions are too scant to support a 

 considerable number of herbivorous quadrupeds. 



634. It were easy for us to multiply examples of these 

 necessary relations between the existence of an animal species 

 in a given locality, and the existence of certain climatic, 

 phytological, or zoological conditions ; but we want space for 

 such details, and the considerations we have just givsn appear 

 to us sufficient to give an idea of the manner by which nature 

 has accomplished the repartition of animal species over dif- 

 ferent points of the surface of the globe ; and to attain the 

 end we proposed in touching on this subject, it only remains 

 for us to take a view (coup d'ceil) of the results brought 

 about by the different circumstances of which we have just 



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